Clean up verify_indexed_attestation

[JustinDrake]

Cosmetic changes:

• Add 4 lines of comments (now every statement has a comment)
• Avoid unnecessary assert (the end goal for me is for asserts to be exclusive to the operation processing helpers).
• Merge returns into one (increase readability, reduce verbosity)
• Use shorter-named bit_0_indices and bit_1_indices helper variables

Substantive change:

• Remove the condition that len(0_indices) + len(1_indices) > 0. This condition is redundant in the context of process_attester_slashing because of slashed_any. It is artificial in process_attestation where validators are incentivised to maximise new attestations.

[djrtwo]

aw, you skipped the raise. Understandable I suppose as it's not anywhere else in the spec.

This looks good to me.

[djrtwo]

It is artificial in process_attestation where validators are incentivised to maximise new attestations

A bit torn on this. Can a validator then include any empty attestations in the extra attestations positions and unnecessarily bloat block size? Seems like it could become an unintended source of graffiti

[JustinDrake]

Seems like it could become an unintended source of graffiti

I've been thinking about this. It turns out there are other cheap ways to generate graffiti. For example, a block proposed can make Transfers where sender == recipient and encode data in the amounts. Because graffiti cannot be avoided the Zen approach is to embrace them. The good news is that all operations are capped, so at a minimum any source of graffiti is bounded.

and unnecessarily bloat block size?

Bloating is negligible, or even zero since the default rational behaviour is probably to max out the number of attestations in a block to maximise revenue. And using Attestations for graffiti means forgoing on the opportunity to make fees, so there's an opportunity cost.

[djrtwo]

added back in the test_empty_aggregation_bitfield test, but changed it to ensure it passes. It's a strange case and because we explicitly (at this time) want it to pass, it's good to have a test for it :)